1. Field of the Invention
The present invention relates to an optical information recording medium comprising a substrate and a light absorption layer formed thereon which is capable of absorbing light and comprises (a) a phthalocyanine compound (I) having a thermal decomposition temperature in a range of 250.degree. C. to 350.degree. C., measured by thermogravimetry with a temperature elevation rate of 10.degree. C., and (b) a phthalocyanine compound (II) having a thermal decomposition temperature in a range of 350.degree. C. to 450.degree. C., measured by the same thermogravimetry as with the phthalocyanine compound (I).
2. Discussion of Background
Recently write once read many type CDs (compact disks) have been actively developed. These CDs, unlike conventional CDs, have a feature that user's information can be recorded and recorded information can be reproduced by commercially available CD players, since such write once read many type CDs meet the standards of the conventional CDs.
A recording and reproduction medium of the above-mentioned write once read many type is proposed in Japanese Laid-Open Patent Application 2-42652, which can be fabricated by spin coating a dye on a substrate to provide a light absorption layer on the substrate, and providing a metal reflection layer on the back side of the substrate opposite to the light absorption layer.
Furthermore, Japanese Laid-Open Patent Application 2-132656 discloses that appropriate selection of the complex index of refraction and the thickness of the light absorption layer provides a compact disk which satisfies the CD standards with respect to recorded signals. Such compact disks are now in fact commercialised as write once read many type CDs.
However, write once read many type CDS using dyes as disclosed in Japanese Laid-Open Patent Applications 2-42652 and 2-132656 do not have sufficient light resistance for use in practice and therefore have the shortcoming that the signal characteristics thereof are caused to deteriorate when exposed to sunrays for an extended period of time, and eventually these CDs become unable to satisfy the CD standards.
The above-mentioned problem is considered to be caused by the deterioration of dyes used therein, in particular, conventionally employed cyanine dyes, when the dyes are exposed to light.
In order to minimize the deterioration of such dyes, there has been proposed the addition of a light stabilizer to the light absorption layer as disclosed in Japanese Laid-Open Patent Application 63-159090. However, in the light absorption layer disclosed in Japanese Laid-Open Patent Application 63-159090, when the light stabilizer is added in a small amount, specifically in an amount of less than 20 wt. % of the entire weight of the light absorption layer, sufficient light resistance for use in practice cannot be attained, while when the light stabilizer is added in a large amount, specifically in an amount of more than 20 wt. %, the optical and/or thermal characteristics of the light absorption layer change and various signal characteristics therefore deteriorate.
Japanese Laid-Open Patent Application 3-62878 discloses a highly heat resistant write once and read many type CD, which employs a highly light resistant phthalocyanine compound. More specifically, in the case of this CD, a particular group is introduced into a phthalocyanine compound which is used in the light absorption layer thereof, and such phthalocaynine compound is spin coated, whereby the complex index of refraction of the light absorption layer is appropriately adjusted so as to be suitable for use in the highly heat resistant write once read many type CD. The write once read many type CD disclosed in the above-mentioned Japanese Laid-Open Patent Application 3-62878, however, has the shortcoming that the quality of regenerative signals is inferior to the quality of regenerative signals output from light absorption layers comprising the conventional cyanine dyes. More specifically, the CD disclosed in the above-mentioned Japanese Laid-Open Patent Application 3-62878 has the shortcoming that the signal length (pit length) of each of signals which constitute regenerative signals is apt to deviate from a theoretical intrinsic value of CD signals, and therefore reproduction errors are apt to occur.